1. Technical Field of the Invention
The present invention relates to a semiconductor device manufactured by a process to anneal a semiconductor film by the use of a laser beam (hereinafter, referred to as laser anneal), and to a method for manufacturing the same. Incidentally, the semiconductor device referred herein includes an electrooptical device, such as a liquid crystal display device and light-emitting device, and an electronic apparatus including such an electrooptical device as a part.
2. Description of the Related Art
In recent years, study has been broadly made on the art to carry out laser anneal on a semiconductor film formed over an insulating substrate of glass or the like in order for crystallization or improving crystallinity. Such semiconductor films often use silicon. In the present description, the means for crystallizing a semiconductor film by using a laser beam and obtaining a crystalline semiconductor film is referred to as laser crystallization. Incidentally, in the description, the crystalline semiconductor film refers to a semiconductor film a crystallized region exists, including a semiconductor film crystallized all over the surface.
The glass substrate is cheap in price and excellent in workability as compared to the conventionally often used synthetic quartz glass substrate, having a merit to easily prepare a large-area substrate. This is the reason of the studies noted above. Meanwhile, the laser is used, by preference, in crystallization because the glass substrate is low in melting point. The laser can deliver high energy only to the semiconductor film without substantially increasing in substrate temperature. Furthermore, throughput is by far high as compared to the heating means using an electric furnace.
Because the crystalline semiconductor film formed through laser anneal has high mobility, thin film transistors (TFTs) can be formed using the crystalline semiconductor film. They are broadly utilized, e.g. in a monolithic liquid-crystal electrooptical device having pixel-driving and drive-circuit TFTs formed on one glass substrate.
Meanwhile, there is preferential use of a method for laser anneal that the high-output pulse laser light, of an excimer laser or the like, is formed through an optical system into a square spot in several-centimeter square or a linear form having a length of 10 centimeters or longer on an irradiation plane in order to scan the laser light (or moving a laser-light irradiation position relatively to the irradiated plane), because of high producibility and industrial superiority.
Particularly, the use of a linear beam can realize laser irradiation over the entire irradiation surface by scanning only in the direction perpendicular to a lengthwise direction of the linear beam, differently from the case using the laser light in a spot form requiring scanning back-and-forth and left-and-right, providing high production efficiency. The scanning in a direction rectangular to the lengthwise direction is carried out because the direction of scanning is the highest in efficiency. Due to the high production efficiency, the use of a linear beam formed of pulse-oscillated excimer laser light through a proper optical system in the current laser anneal process is in the mainstream of the technology to manufacture liquid crystal display devices using TFTs.
However, the crystallization process by laser light irradiation causes to form a multiplicity of convexes (ridges) in the surface of an obtained crystalline semiconductor film, lowering film quality. Namely, when laser light is irradiated to a semiconductor film, the semiconductor film instantaneously melted to cause local expansion. The internal stress caused by the expansion is relaxed to thereby form ridges in the surface of the crystalline semiconductor film. The height difference of ridges is nearly 0.5 to 2 times the film thickness.
In the insulated-gate semiconductor device, the ridges in the crystalline semiconductor film surface have a potential barrier or trap level formed due to dangling bond or lattice deformation, increasing the interface level between the active layer and the gate dielectric film. Meanwhile, the ridge at its summit is sharp and readily causes electric field concentration to possibly act as a source of current leak, eventually causing dielectric breakdown and short circuit. In addition, the ridges in the crystalline semiconductor film surface hinder the coverage of a gate dielectric film deposited by a sputter or CVD process, reducing reliability, e.g. poor insulation. Meanwhile, the factor determining electric-field effect mobility of a TFT includes a surface-scattering effect. The planarity in the interface of an active layer and a gate dielectric film of the TFT has a great effect upon the electric-field effect mobility. As the interface is greater in planarity, the higher electric-field effect mobility is available without undergoing the affection of scattering. In this manner, the ridges in the crystalline semiconductor film surface give effects upon every TFT characteristic, changing even the yield.
It is an object of the present invention to provide a method for forming a semiconductor film having a surface which is reduced in ridge and manufacturing a semiconductor device using such a semiconductor.